In a digitally controlled printing system, for example an inkjet printing system, a print media is directed through a series of components. The print media can be a cut sheet or a continuous web. A web or cut sheet transport system physically moves the print media through the printing system. As the print media moves through the printing system, marks are controllably made on the print media by one or more printheads, which are typically not in contact with the print media, to form the desired image or pattern.
For printing a color image, the printing system can have a plurality of printing stations, each having a printhead for printing one of the color channels (e.g., cyan, magenta, yellow and black) that make up the color image. If suitable color-to-color registration is not maintained in the printing system, print defects such as color halos at the edges of multicolor features can be seen.
Similarly, functional printing of devices can be done in multiple successive steps using a plurality of printing stations. If suitable registration is not maintained between printing stations, the performance of the printed device can be degraded. In fact, the desired registration tolerances for functional printing can be tighter than what is required for color image printing.
One approach for achieving registration of patterns printed by different printheads on a web of media is to use in-situ measurement techniques on the printed web such that the registration can be monitored and controlled to be within a required tolerance. Registration marks can be printed on the web of media at the same time as each color layer of the image is printed. The registration marks can be monitored by a control system and appropriate adjustments can be made to the printing process. For example, registering a pattern along the web motion direction (also called the in-track direction) that is being printed by a second digital printhead to a pattern that was printed previously by first digital printhead can be done by controlling the timing of the marking process of the second digital printhead. For example, for inkjet printheads the timing of the jetting of the ink drops by the second printhead can be advanced or delayed as needed.
Although methods exist for registering portions of the print that are successively printed by different printheads, what is needed for precision printing is to design the web transport for a roll-to-roll digital printing system in such a way that the size of registration errors introduced in the printing system is reduced.